The bills referred to are not part of any 'stimulus package'. They are in fact independent bills submitted in parallel in the House and Senate. The program is not any kind of mandatory thing - it's a voluntary voucher program to encourage people to get rid of inefficient vehicles and move on to more efficient ones.

The text of the two bills (which, as far as I can tell, are identical) can be accessed at:

If you take the time to review the actual bills you'll see they're describing a voluntary incentive program to motivate people to surrender gas-guzzlers and get a voucher toward purchase of a more fuel-efficient vehicle.